1. Field of the Invention
The present invention relates to a process for post-treating titanium dioxide pigment.
2. Description of the Prior Art
Solutions of salts of tetravalent titanium are often used for post-treatment, e.g. titanyl sulphate solutions. Titanyl sulphate solutions, however, have the disadvantage of tending to be unstable. In such solutions, when allowed to rest, the content of dissolved titanium often decreases and sediments are formed. This in turn results in a loss of active substance. Moreover, changes take place in the solution which may lead to variations in the pigment properties that were to be improved by the post-treatment. A further drawback of titanyl sulphate solutions is the fact that their production, and hence their use, is only economically justified if intermediate products are available from the TiO.sub.2 pigment production by the hydrolysis of titanyl sulphate solutions in the so-called sulphate process.
Another alternative suggests the use of other compounds of tetravalent titanium for post-treatment, e.g. chlorides as in U.S. Pat. No. 3,146,119. These chlorides of titanium are employed in the form of a solution hereinafter called "titanium oxide chloride solution". Such a "titanium oxide chloride solution" is not intended to mean the solution of the stoichiometric compound TiOCl.sub.2, but rather an aqueous solution containing dissolved titanium and hydrochloric acid characterized by its contents of TiO.sub.2 and HCl, leaving open which of the numerous potential products of the hydrolysis of titanium tetrachloride are actually present in the solution.
The titanium oxide chloride solution is usually made by dissolving titanium tetrachloride in water or dilute hydrochloric acid-containing aqueous solutions. Preparation of this titanium oxide chloride solution is relatively expensive since elemental chlorine must be used in general to produce titanium tetrachloride. Moreover, TiCl.sub.4 is produced at elevated temperatures, and expensive purification steps are required to free titanium tetrachloride from foreign metals, specifically iron, aluminum, vanadium, chromium, etc. As properties, particularly the optical properties, of the titanium dioxide pigment are affected by foreign metals, which form colored ions, the titanium oxide chloride solution was preferably made from the purest titanium tetrachloride as is used for the manufacture of TiO.sub.2 pigments by reaction with oxygen in the "chloride process".